By Dan May
On Our Land
Dan May
I always enjoyed teaching a course in physical geography that catered to non-science college students seeking to meet one of their general education requirements. Physical geography is not about arbitrary political boundaries and map labels, but about the features that arise from the interaction of the atmosphere with the solid Earth. It includes topics such as weather, climate, the water cycle and soil, as well as physical attributes of ecosystems and their correlation with biomes.
One tool students use is called a climograph, which is a graphical chart of average annual monthly temperatures and precipitation for a given setting. Climate graphs are available for nearly anywhere in the world at en.climate-data.org/. When scheduling an overseas trip, it is often one of the first information sources that I consult.
Temperature and precipitation are not the only things that change seasonally. We often forget to ponder all the sensory features of each season. Everyone is enjoying the recent warm, sunny weather here after a cold dreary start to spring, and the welcome burst of greenery and flowering plants is a delight along with the sounds of songbirds and frogs.
Less often considered are the outdoor scents of a season. I enjoy the smell of woodsmoke in winter, but spring’s odors are comparatively intoxicating. Flowering magnolias, dogwoods and lilacs smell as nice as they look. Thus many flowers end up as components of perfumes and colognes. Even the first cuts of spring lawns smell amazing.
We measure and report particulate pollen amounts during the growing season, and for those suffering from allergies, loosely describe their sources as trees, grasses, weeds, etc. However, I would love to see a “scentograph” that would show the extent and variety of aromatic vapors that linger in the air in any given season.
Many atmospheric odors originate with flowering plants, and their scents attract pollinating insects and a few birds. However, odors also originate from the microbiota living in soils and are products of their subsurface metabolism.
I first became familiar with soil odors when working on environmental remediation projects where soils unfortunately had been contaminated with spilled or leaked fuels and solvents, many of which include aromatic volatile organic compounds. Aromatic means we can smell them, and volatile means that they evaporate readily from liquid to gaseous states.
Field instruments can detect most organic vapors, but the human nose is amazingly sensitive too. It is easy to recognize diesel from gasoline by odor, even at very low concentrations. And although I strongly discourage it, a whiff can distinguish whether the toxic anti-knock additives used in gasoline over the years are from tetraethyl lead, benzene and toluene, methyl tert-butyl ether or ethanol.
The odors that arise naturally from soil microbiota are particularly abundant in the spring and early summer when the pace of life picks up as the soil warms and bacteria, fungi, insects and worms get to work digesting last autumn’s leaf and plant litter, along with each other. The earthy smell of bagged topsoil or a handful of freshly tilled garden soil are largely their respiratory or otherwise excreted products.
Most soil gases are odorless, like those in the atmosphere – nitrogen, oxygen and carbon dioxide. But some are pungent, like ammonia or hydrogen sulfide. And dozens of aromatic hydrocarbons are produced in bio-decay. Soil gases are useful indicators of what is living in the soil, and the systematic investigation of soil gases is a focus area of environmental research today.
Much of the current interest in soils is due to the observation that there is nearly three times as much total carbon present in soil as there is in all living things. Most of this eventually leaks out as carbon dioxide, methane or VOCs, but some is entrained (and permanently sequestered) in bacterially-precipitated minerals. A research race is underway to see which bacteria can be identified, cultured and/or engineered to capture more carbon as mineral matter in soil instead of adding carbon dioxide to the atmosphere.
Growing up, my mother would daily remind me to wash the dirt off my hands when I came inside. She would undoubtedly be aghast to see me sniff a handful of soil to crudely classify it as sweet, sour, musky or acrid. But scentographs for soils are being developed, and hopefully it won’t be too long before we also know what’s really in the air above ground when cherry blossoms are blooming.
Dan May is a local geologist.